共查询到20条相似文献，搜索用时 15 毫秒
1.
2.
Partial melting of ultrahigh-pressure metamorphic rocks at convergent continental margins: Evidences,melt compositions and physical effects 总被引：2，自引：0，他引：2
Ultrahigh-pressure(UHP) metamorphic rocks are distinctive products of crustal deep subduction,and are mainly exposed in continental subduction-collision terranes. UHP slices of continental crust are usually involved in multistage exhumation and partial melting, which has obvious influence on the rheological features of the rocks, and thus significantly affect the dynamic behavior of subducted slices. Moreover,partial melting of UHP rocks have significant influence on element mobility and related isotope behavior within continental subduction zones, which is in turn crucial to chemical differentiation of the continental crust and to crust-mantle interaction.Partial melting can occur before, during or after the peak metamorphism of UHP rocks. Post-peak decompression melting has been better constrained by remelting experiments; however, because of multiple stages of decompression, retrogression and deformation, evidence of former melts in UHP rocks is often erased. Field evidence is among the most reliable criteria to infer partial melting. Glass and nanogranitoid inclusions are generally considered conclusive petrographic evidence. The residual assemblages after melt extraction are also significant to indicate partial melting in some cases. Besides field and petrographic evidence, bulk-rock and zircon trace-element geochemical features are also effective tools for recognizing partial melting of UHP rocks. Phase equilibrium modeling is an important petrological tool that is becoming more and more popular in P-T estimation of the evolution of metamorphic rocks; by taking into account the activity model of silicate melt, it can predict when partial melting occurred if the P-T path of a given rock is provided.UHP silicate melt is commonly leucogranitic and peraluminous in composition with high SiO_2,low MgO, FeO, MnO, TiO_2 and CaO, and variable K_2 O and Na_2 O contents. Mineralogy of nanogranites found in UHP rocks mainly consists of plagioclase + K-feldspar + quartz, plagioclase being commonly albite-rich.Trace element pattern of the melt is characterized by significant enrichment of large ion lithophile elements(LILE), depletion of heavy rare earth elements(HREE) and high field strength elements(HFSE),indicating garnet and rutile stability in the residual assemblage. In eclogites, significant Mg-isotope fractionation occurs between garnet and phengite; therefore, Mg isotopes may become an effective indicator for partial melting of eclogites. 相似文献
3.
部分熔融与高级变质岩流变机制——以内蒙古大青山高级变质岩为例 总被引：5，自引：0，他引：5
部分熔融作用与高级变质岩变形作用是相互制约,变形作用能够提高岩石部分熔融程度,降低熔融温度。熔体存在影响和制约岩石强度和变形机制。大青山高级岩经历了下部地壳构造层次变质变形和深熔作用改造,形成了复杂构造要素组合。宏观与微观构造特点表明:高级变质岩变形机制主要为熔体增强颗粒边界扩散和颗粒流动,使岩石发生大规模的塑性流动。在宏观上形成了不对称流动组构、熔融线理、岩石和矿物条带、层内底辟褶皱和大型穹窿构造。但是,在微观上矿物颗粒变形不明显,晶内变形组构不发育,表现为三边平衡结构,与静态结晶变质岩结构相似,形成了地壳深部构造层次上变质构造岩-构造片麻岩。 相似文献
4.
高级变质-深熔作用中伴随有熔体的形成,在缺失流体的不一致熔融条件下,如果熔体的萃取不完全,原地结晶熔体与残留体之间可发生特殊的退变反应———逆(熔)反应,这种反应与进变质脱水熔融恰好相反。逆(熔)反应的最佳判断标志就是在浅色体和早期深熔过程中形成的不一致熔融相之间含水矿物组合的生长。逆(熔)反应形成的退变组合是深熔作用过程的一部分,而不是另外期次的变质事件叠加。逆(熔)反应的地质意义在于它可能会影响到熔体和流体组成、流体-岩石的相互作用以及对质量平衡的研究,尤其是对p-T路径恢复的影响,从而影响到构造和热流模型的推断。 相似文献
5.
Spatially-focussed melt formation in aluminous metapelites from Broken Hill, Australia 总被引：6，自引：1，他引：6
Large garnet poikiloblasts hosted by leucosome in metapelitic gneiss from Broken Hill reflect complex mineral–melt relationships. The spatial relationship between the leucosomes and the garnet poikiloblasts implies that the growth of garnet was strongly linked to the production of melt. The apparent difficulty of garnet to nucleate a large number of grains during the prograde breakdown of coexisting biotite and sillimanite led to the spatial focussing of melting reactions around the few garnet nuclei that formed. Continued reaction of biotite and sillimanite required diffusion of elements from where minerals were reacting to sites of garnet growth. This diffusion was driven by chemical potential gradients between garnet‐bearing and garnet‐absent parts of the rock. As a consequence, melt and peritectic K‐feldspar also preferentially formed around the garnet. The diffusion of elements led to the chemical partitioning of the rock within an overall context in which equilibrium may have been approached. Thus, the garnet‐bearing leucosomes record in situ melt formation around garnet porphyroblasts rather than centimetre‐scale physical melt migration and segregation. The near complete preservation of the high‐grade assemblages in the mesosome and leucosome is consistent with substantial melt loss. Interconnected networks between garnet‐rich leucosomes provide the most likely pathway for melt migration. Decimetre‐scale, coarse‐grained, garnet‐poor leucosomes may represent areas of melt flux through a large‐scale melt transfer network. 相似文献
6.
Magmatic diversity of western Mexico as a function of metamorphic transformations in the subducted oceanic plate 总被引：1，自引：0，他引：1
Arturo Gómez-Tuena Laura Mori Ofelia Pérez-Arvizu 《Geochimica et cosmochimica acta》2011,75(1):213-241
This geochemical study of the Mexican subduction zone elucidates how metamorphic and dehydration reactions affecting the subducted oceanic plate at different depths can influence magmatic diversity. In the western Trans-Mexican Volcanic Belt, there is a narrow potassic volcanic front running in parallel to the Middle American Trench that becomes replaced by intraplate-like high-Nb rocks to the north, and by more typical calc-alkaline products to the southeast. Potassic rocks have high MgO and are enriched in incompatible trace elements, but have lower heavy rare earth element contents than more evolved calc-alkaline and high-Nb magmas, and slightly more enriched Sr, Nd and Pb isotopes. Potassic magmas also have higher Rb/Cs and Ba/Cs ratios than the calc-alkaline and high-Nb suites, and extend to unusually high Nb/Ta ratios that correlate positively with Rb/Ta, Zr/Ta, La/Ta and Gd/Yb. These chemical variations are inconsistent with different extents of melting of a peridotitic source, but are also incompatible with melting of a phlogopite-rich mantle (vein-plus-wall-rock relationship), unless mica is totally consumed during melting, and a titaniferous phase such as rutile remains in the residue together with garnet. This assemblage is unlikely in the source region of primitive hydrous magmas, but it is what would be expected during dissolution of phengite and monazite/allanite in the subducted slab, with the concurrent formation of an anhydrous rutile-bearing eclogite. The magmatic diversity of western Mexico can thus be explained by invoking contributions of chemically different subduction agents as a function of slab depth and residual mineralogy: a low-pressure/temperature aqueous fluid would induce melting of the peridotitic mantle wedge and form typical calc-alkaline volcanoes, whereas a deeper and hotter slab-derived melt (or supercritical liquid) would contribute to the formation of potassic magmas due to phengite/monazite/allanite disintegration. In this context, intraplate-like magmas derive from decompression melting of the upper mantle as a natural consequence of subduction geodynamics. 相似文献
7.
Silica depleted melting of pelites. Petrogenetic grid and application to the Susqueda Aureole, Spain 总被引：2，自引：0，他引：2
Metapelitic rocks in the low pressure contact metamorphic aureole around the Susqueda igneous complex, Spain show a number of features that make them an ideal testing ground for the modelling of silica‐undersaturated melting. Rocks in the aureole experienced localized depletion in silica by the segregation of quartz veins during a pre‐anatectic, regional cordierite‐andalusite grade metamorphic event. These rocks were then intruded by gabbroic to dioritic rocks of the Susqueda igneous complex that formed a migmatitic contact metamorphic aureole in the country rocks. This migmatisation event caused quartz‐saturated hornfels and restite formation in rocks that had experienced no quartz vein segregation in the previous regional metamorphic event, but silica‐undersaturated melting in those rocks that were previously depleted in silica. Silica‐undersaturated melting is investigated using a new petrogenetic P–T projection and equilibrium pseudosections calculated in the KFMASH and NCKFMASH systems, respectively. The grid considers quartz absent equilibria and a range of phases that form typically in silica‐undersaturated bulk compositions, for example corundum. It is shown that the quartz‐rich precursors in the Susqueda contact aureole produced about 10% melt during contact metamorphism. However, most of this melt was extracted leaving behind rocks with restitic bulk compositions and minor leucosome segregation. It is suggested that the melt mixed with the host igneous rocks causing an apparent magmatic zoning from diorite in the centre of the complex to tonalite at the margins. In contrast, the quartz‐poor precursors (from which the quartz veins segregated) melted in the silica‐undersaturated field producing a range of assemblages including peritectic corundum and spinel. Melting of the silica‐undersaturated rocks produced only negligible melt and no subsequent melt loss. 相似文献
8.
The St. Malo migmatitic dome represents an interesting example wherein migmatites arise from the anatexis of the surrounding gneisses. Petrographical and chemical data suggest that leucosome compositions are compatible with partial melting of the quartzo-feldsphathic fraction of the parent gneiss. The contribution of the incongruent melting of biotite to the melt does not exceed 5% of the parent rock.Petrogenetic modelling based on experimental data and assuming non modal batch melting show that the K, Rb, Ca, Sr, U and Th chemical patterns of these migmatites result in fact from the interaction of several mechanisms, namely: equilibrium partial melting, mixing between melts and refractory minerals (biotite and accessories), melt removal and late hydrothermal alteration. Zr, Y and Th which are mostly hosted in accessory minerals are significantly withheld from the melts and remain stored in melanosomes (metatexites) except when leucosomes are affected by mixing (diatexites). U is frequently enriched in the leucosomes as well as in some melanosomes suggesting external supply. 相似文献
9.
Geochemistry and petrogenesis of Sikait leucogranite, Egypt: an example of S-type granite in a metapelitic sequence 总被引：1，自引：0，他引：1
The Sikait leucogranite (SLG) is a body of porphyritic garnet granite intruded a metapelitic sequence and interlayered orthogneisses.
Multiple deformation and low- to medium-grade metamorphism of the sequence was closely associated with pluton emplacement.
Textural features of the SLG indicate that subsolidus plastic deformation was induced during the extensive thrusting. The
granite is strongly peraluminous, alkali-rich and HFS (high field-strength) elements-depleted with low contents of REE, all
facts that substantiate the geochemical characteristics of S-type granites.
The geologic and geochemical features are consistent with a dehydration melting model of the hosted metapelites to generate
the peraluminous SLG. However, geochemical modelling of trace elements and REEs suggest that the anatectic partial melt was
subsequently affected by fractional crystallization of feldspars. This could explain much of the chemical attributes of SLG.
Received: 20 September 1994 / Accepted: 2 August 1996 相似文献
10.
Textural evolution and partial melting of arkose in a contact aureole: a case study and implications
Recognition of partial melting in metamorphic rocks is a difficult task, as leucosomes can have a variety of origins. By comparing the observed values of the solid-solid dihedral angles with the known equilibrium values, and close examination of the shapes and compositions of feldspar grains, it is possible to unequivocally identify melt textures. Textural relations in a series of meta-arkose samples from the contact aureole of the Ballachulish Igneous Complex in the Scottish Highlands demonstrate that, when former melt pockets are not highly deformed, their presence can be recognized petrographically, by detailed examination of textures on the grain scale. Identification of melt textures and their distribution in the Ballachulish aureole has led to appreciation of the fundamental role of magmatically derived H2O in producing the partial melting. It has also allowed calculation of the H2O flux involved, and recognition that fractures were the major fluid pathways during metamorphism. 相似文献
11.
高级变质岩中的熔体包裹体为包裹于寄主矿物（石榴子石、锆石等）中的熔体滴,主要由子晶矿物、玻璃和一些偶然捕获的固相矿物组成.最近10年来,熔体包裹体已经成为研究区域变质岩部分熔融作用的新手段和新途径.熔体包裹体粒径通常 < 20 μm,微小的尺度对熔体包裹体识别与分析提出巨大挑战.为此系统总结了熔体包裹体的识别方法、结构特征、化学成分分析方法,并指出熔体包裹体研究对于部分熔融的识别、部分熔融发生的温压条件、机制和流体状态的限定等具有重要的意义. 相似文献
12.
变形变质岩熔融及其与化学成分的关系研究 总被引：1，自引：0，他引：1
高温高压熔融实验结合岩石化学分别研究表明：在无水体系中，变形变质岩的初熔温度随ＦｅＯ＋ＭｇＯ＋ＣａＯ＋ＴｉＯ２、Ａｌ２Ｏ３等的增高而增高；随ＳｉＯ２、Ｋ２Ｏ＋ＮａＯ含量的增高而降低。此外，岩石的变形变质程度、压力也对岩石的初熔产生重大影响。本文还初步探讨了ＲＥＥ与岩石熔融的关系。金属元素在熔体与残留体的分配研究表明：常量金属元素在熔体中相对富集，在熔体与残留体之间的分异度（│ｌｇＤｉ│随ＳｉＯ２的 相似文献
13.
Partitioning of elements between majorite garnet and melt and implications for petrogenesis of komatiite 总被引：5，自引：1，他引：5
Eiji Ohtani Iwao Kawabe Junko Moriyama Yoshihiko Nagata 《Contributions to Mineralogy and Petrology》1989,103(3):263-269
Partitioning of elements between majorite garnet and ultrabasic melt has been studied at 16 GPa and 1950° C. Ca, Ti, La, Sm, Gd, Zr, Hf, Fe, Ni, Mn, K, and Na are enriched in the melt, whereas Al, Cr, V, Sc and Yb are concentrated in majorite garnet. Thus, majorite garnet fractionation by partial melting could produce chemical heterogeneities in these elements deviating from chondritic abundance. Using the partitioning behaviour of elements between majorite garnet and ultrabasic melt, the petrogenesis of komatiite is discussed. A simple model to explain the chemical varieties of komatiites is as follows. Aluminadepleted komatiite was generated by partial melting of the primitive mantle at 200–650 km depth, and alumina-enriched komatiite is the product of remelting of the residual solid at the same depths, whereas alumina-undepleted komatiite was formed by partial melting of the primitive upper mantle at depths shallower than 200 km. We suggest the possibility of large-scale chemical layering or heterogeneity in the early Archean upper mantle as an alternative model for komatiite genesis; shallower mantle depleted in majorite garnet and the underlying mantle enriched in majorite garnet. Alumina-depleted and alumina-enriched komatiites in the early Archean might be generated by a high degree of partial melting of the layered mantle. Such chemical layering could have been homogenized by the late Archean. This explains the observations that alumina-depleted and alumina-enriched komatiites were generally formed in the early Archean but alumina-undepleted komatiite was erupted in the late Archean. 相似文献
14.
W.S. Fyfe 《Tectonophysics》1973,17(3):273-283
The formation of the melts which produce intrusions of the granite family are considered to result from the partial fusion of high-grade metamorphic rocks. The melting behaviour of such materials is considered. Such melts will rarely be water-saturated and the degree of water-saturation must set limits on the ability of the melt to rise. The natural residue of fractional fusion will be metamorphic rocks of the granulite facies. The motion of the melts seems reasonable in relation to the theory of Taylor instability and Stokesian rise of the materials. Differences between Archaean batholith patterns and those in modern belts of plate subduction will be considered. It is suggested that such differences could result from fusion processes in the upper mantle occurring at higher levels than at the present time and “ocean ridges” being more closely spaced. 相似文献
15.
区域变质作用中的流体 总被引：8，自引：0，他引：8
区域变质条件下流体的流动有 4种标志 :( 1)细脉 ;( 2 )岩石学 ;( 3 )稳定同位素 ;( 4 )常量元素的交代作用。不同级别的区域变质作用中 ,流体影响着岩石的变质反应和变形 ;在高级变质的情况下甚至有熔体出现。在超高压变质条件下 ,流体量比地壳范围区域变质要少得多 ,从大别山超高压变质带的资料可知 ,流体的演化有明显的阶段性 ,局部曾发现熔融包裹体。水流体的介入 ,引起岩石的退变质和元素地球化学变异 ,是超高压变质岩抬升、进入中下地壳的产物。新近的实验岩石学成果说明 ,多硅白云母、角闪石等含羟基的矿物 ,在俯冲达 10 0km以下依然稳定 ,而一些花岗岩体系在超高压的条件下产生的超临界流体 ,乃是花岗岩、片麻岩只能部分保留超高压矿物组合的原因。 相似文献
16.
Progress in development of thermodynamically based models ofsilicate equilibria with explicit entropy budgets has motivateda reexamination of the conclusion of McKenzie (Journal of Petrology25, 713–765, 1984) that isentropic upwelling sufficesas a model of mantle melting. An entropy budget equation forfractional melting with melt migration in an upwelling two-phasecontinuum is presented. The energetically self-consistent meltproduction model predicted by MELTS is used to evaluate numericallythe magnitudes of differences between fractional melting (withmelt migration) and equilibrium melting (without relative movement)that can be bounded in one dimension: chemical advection byout-of-equilibrium melt; thermal disequilibrium between migratingliquid and residue; frictional dissipation of gravitationalpotential; dissipation as a result of solid compaction. Likethe familiar isobaric case in which fractional melting is significantlyless productive than equilibrium melting, chemical isolationof the escaping melts from the residue reduces the oceanic crustalthickness by 相似文献
17.
A. C. Ganzhorn L. Labrousse G. Prouteau C. Leroy J. C. Vrijmoed T. B. Andersen L. Arbaret 《Journal of Metamorphic Geology》2014,32(6):647-673
Evidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnet that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (i) as zoned segregations found in high‐P (ultra)mafic bodies; (ii) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss; and (iii) as undeformed pegmatites cutting the main Caledonian structures. Segregations post‐date the eclogite facies foliation and pre‐date the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation, and undeformed pegmatites are post‐kinematic and were formed at the end of the deformation history. The geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca‐poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K‐poor compositions corresponds to segregation hosted in (ultra)mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward‐buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800 °C implies the presence of an H2O‐rich fluid phase at great depth to facilitate H2O‐present partial melting. 相似文献
18.
N. H. S. Oliver P. J. Pearson R. J. Holcombe A. Ord 《Australian Journal of Earth Sciences》2013,60(3):467-484
The Mary Kathleen U‐REE orebody of the Proterozoic Mt Isa Block was the product of chemical and physical interaction between regional metamorphic/hydrothermal fluids and preexisting calcic skarns. The deposit provides excellent examples of mechanical control on ore localisation and of the complexity of ores in rocks with protracted thermal histories. Host skarns were produced by contact metasomatism around the 1740 Ma Burstall Granite, whereas the allanite‐uraninite ore formed under amphibolite‐facies conditions, late during the D2 phase of the ca 1550–1500 Ma Isan orogeny. Observations of ore geometry are consistent with previous geochronologic data demonstrating a large time gap between skarn formation and ore genesis. Numerical modelling of coupled deformation and fluid flow suggests that veins at the core of ore shoots may have formed as tensile or shear fractures during coupling of the competent skarn host with the late‐D2 Mary Kathleen Shear Zone, allowing a change of orientation of ore shoots with distance from the shear zone. Mineral chemistry and petrographic observations suggest the possible role of a redox control on chemical localisation of ore by conversion of Fe2+‐rich clinopyroxene‐rich skarn host to Fe3+‐rich secondary garnet ‘skarn’ and uraninite‐allanite ore. Alternately, fluid pressure drops as a consequence of fracturing of the host skarn may have triggered fluid unmixing, or fluid mixing, leading to ore precipitation. Available data do not allow clear definition of the ultimate source of the U and REE, nor of the specific chemical ore‐forming mechanism. However, regional constraints, previous Sm–Nd modelling, and our numerical models suggest a combination from proximal skarn hosts and from distal sources accessed by flow of metamorphic and/or late tectonic igneous‐derived fluids. The deposit has some similarities with ironstone‐hosted Cu–Au ± U deposits found in the nearby Cloncurry Belt. 相似文献
19.
20.
Silvio Mollo Valeria Misiti Piergiorgio Scarlato 《Central European Journal of Geosciences》2010,2(2):188-198
We experimentally investigate the major and trace elements behavior during the interaction between two partially molten crustal rocks (meta-anorthosite and metapelite) and a basaltic melt at 0.5–0.8 GPa. Results show that a hybrid melt is formed at the basalt-crust contact, where plagioclase crystallizes. This contact layer is enriched in trace elements which are incompatible with plagioclase crystals. Under these conditions, the trace element diffusion coefficients are one order of magnitude larger than those expected. Moreover, the HFSE diffusivity in the hybrid melt is surprisingly higher than the REE one. Such a feature is related to the plagioclase crystallization that changes the trace elements liquid-liquid partitioning (i.e. diffusivity) over a transient equilibrium that will persist as long as the crystal growth proceeds. These experiments suggests that the behaviour of the trace elements is strongly dependent on the crystallization at the magma-crust interface. Diffusive processes like those investigated can be invoked to explain some unusual chemical features of contaminated magmatic suites. 相似文献